Dark Origins on Vesta

In this Dawn FC (framing camera) image, a number of small dark areas, mostly clustered in the center and left of the image, are visible in Vesta’s cratered landscape. A lot of these dark patches are small impact craters, which may have excavated dark material from a shallow subsurface layer of Vesta. One of these small craters, in the left middle of the image, features dark rays. This is unusual as rays from impact craters are generally of higher albedo (eg. brighter) than the surrounding surface. This landscape is dominated by two large bowl-shaped fresh scarp rimmed craters, which are approximately 10-20 km in diameter. Bright material is seen slumping into these craters, generally from their rims. Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

NASA’s Dawn mission launched in 2007 and arrived at the protoplanet Vesta in 2011. When the spacecraft returned images of Vesta, which orbits within our solar system’s asteroid belt, scientists spotted dark patches of material scattered across the surface.

It looked as if the dark areas might be the result of impacts that excavated materials from beneath the surface. Now, scientists from the Max Planck Institute for Solar System Research have uncovered the origin of this material. Their study was published in the journal Icarus.

When studying data from the Dawn’s framing camera (FC), the team identified the mineral serpentine in the dark patches of material. Serpentine is a well-known mineral and scientists know the range of temperatures and pressures in which it forms. This means that the team was able to determine when and how serpentine made an appearance on Vesta.

Serpentine cannot survive temperatures over 400 degrees Celsius. Large bodies like Vesta (and the Earth) began their lives balls of molten material, and they stayed hotter than 400 C for long periods of their early history. Because of this, serpentine could not have originated on the protoplanet. Smaller asteroids, on the other hand, cooled relatively quickly after their formation, and this is where the research team believes the serpentine came from. Asteroids containing serpentine would have struck Vesta at fairly slow speeds, creating impacts that did not generate too much heat for the serpentine to survive.

The Numisia crater just south of Vesta’s equator has a diameter of 30 kilometers. Images obtained by the camera system on board NASA’s spacecraft Dawn with the clear filter (left) show dark material in the crater walls and in the material ejected during impact. The camera system’s color filters can filter individual wavelengths from the reflected light and thus make further variations in the surface composition visible (right). In data like this the researchers found the characteristic fingerprints of the mineral serpentine. Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

The team determined that a low-speed impact at a specific angle could have resulted in the distribution of serpentine we see on the surface of Vesta today. They tracked the event to a large impact basin in the protoplanet’s southern hemisphere.

Studying Vesta can help astrobiologists understand how rocky bodies form and evolve. This information can help scientists determine the conditions that lead a rocky world to become either habitable, or simply a dead ball of material drifting through space.

NASA’s Dawn Defines Vesta’s Role in Solar System History. Credit: NASA (YouTube)

As our solar system was forming, Vesta was on its way to becoming a full-fledged planet. At some point in its early evolution, Vesta got stuck. Too big to be an asteroid, but too small to be a planet – Vesta is now referred to as a protoplanet.

Where is Dawn now? A simulation of the spacecraft’s trajectory. Credit: Gregory J. Whiffen, JPL

Dawn is now well on its way to a second large body in the asteroid belt named Ceres. The spacecraft is less than a year away from the object, where it will use its propulsion system to spiral downward to lower and lower altitudes above Ceres’ surface. To read more about the Ceres portion of Dawn’s mission, click here.